1. Field of the Invention
This invention relates to improvements in an automatic focus adjusting apparatus for receiving the object light passed, for example, through a photo-taking lens to thereby detect the defocus state of said photo-taking lens and accomplish focus adjustment.
2. Related Background Art
In an apparatus of this type, when effecting the focus adjustment of the photo-taking lens, the amount of defocus has first been detected, and then the lens driving amount has been calculated as shown in the following equation so that the amount of defocus is "0", and lens driving control has been effected on the basis thereof. EQU Lens driving amount=(amount of defocus)/(sensitivity of lens)
Here, if the distance between an object to be photographed and the camera is constant, focusing can always be continued by effecting the lens driving control as described above. However, during a photographing condition in which the distance between the object to be photographed and the camera is always changing, for example, during a photographing condition in which the object to be photographed is moving toward the camera at a predetermined speed, the distance between the object to be photographed and the camera changes during the time from the start of distance measurement until the completion of lens driving (hereinafter referred to as the AF time-lag) and the lens is out of focus by that amount. Even if the lens has been in focus until that point of time, there has been the disadvantage that the lens is likewise out of focus during the time from the closing of the release switch until the movement of the shutter curtain (hereinafter referred to as the release time-lag). So, apparatuses provided with means for compensating for the follow-up delay by said AF time-lag or said release time-lag are disclosed in Japanese Laid-Open Patent Application Nos. 62-125311, 62-139511 and 62-139512.
The follow-up delay compensating system in the aforementioned propositions will hereinafter be described with reference to FIG. 9(a) of the accompanying drawings. In FIG. 9(a), the ordinate represents the imaging plane position (the imaging plane position of the object to be photographed) and the abscissa represents time.
Here, if the distance between the object to be photographed and the camera changes at all times and the imaging plane position changes as indicated by the solid line in the figure (it assumes the curve as indicated in FIG. 9(a) when the object to be photographed is moving toward the camera at a predetermined speed), the object to be photographed will move during the AF time-lag and out-of-focus of DF2, DF3 and DF4 will always occur. So, if for example, the current amount of defocus DF4 and the preceding amount of defocus DF3 are in the relation that DF4&gt;DF3, it is judged that a follow-up delay is occurring, and the compensation amount is calculated from the following equation: EQU Compensation amount=DF4-DF3 (1)
However, in the photographing of an ordinary moving object which assumes the curve as indicated in FIG. 9(a), even if the compensation as previously described is effected, a follow-up delay corresponding to the amount of defocus DF5 exists and thus, complete compensation is not effected. Also, during the photographing, the follow-up delay by the release time-lag is added to further increase the amount of defocus. The ideal lens position upon consideration of such release time-lag is indicated by the broken line in the figure, and it is more advanced in phase by an amount corresponding to the release time-lag than the change in the actual position of the imaging plane. Accordingly, if the release operation can be entered at a position on this broken line, the lens will be in focus during the photographing.
Here, as an example in which the follow-up delay by the release time-lag is also compensated for by only the detected amount of defocus DF, the AF time-lag is defined as AFTL, the release time-lag is defined as LETL, the sensitivity of the lens is defined as LB and the lens driving amount DL is considered as ##EQU1## and if on the assumption that LETL=AFTL/2, this is substituted into the above equation, the movement of the lens will be as shown in FIGS. 9(b) and 9(c) of the accompanying drawings (FIG. 9(c) shows the time of photographing a moving object differing from that in FIG. 9(b)). That is, the amount of defocus in the same direction as the amounts of defocus DF2 and DF3 is detected, and when the follow-up compensation is judged to be necessary, the follow-up compensation is entered and the lens driving amount is changed to 2.5 DF.
From these figures, it is seen that in FIG. 9(b), the compensation amount is deficient for the ideal position even if such compensation is effected, and in the case of FIG. 9(c), in the first compensation, the ideal position is provided by this compensation. However, in the second and subsequent compensations, a deficient compensation amount or, as shown, a reverse effect (which is due to the fact that the value of the amount of defocus DF4 becomes reverse (positive or negative sign) in direction to the preceding amounts of defocus DF2 and DF3 for the actual position of the imaging plane) is provided and thus, the movement of the lens becomes very unstable and in some cases, there is the danger of the movement of the lens diverging.
With such a system for monitoring and compensating for only the amount of defocus DF, it has been difficult to accomplish ideal lens driving control.